The present invention relates to a two-color thermosensitive recording material utilizing coloring reactions between leuco dyes and color developers, by which two different colors are formed on the recording material.
A conventional thermosensitive recording material comprises a support material such as a sheet of paper and a thermosensitive coloring layer formed on the support material, on which thermosensitive coloring layer colored images can be formed by application of heat thereto. For the heat application for image formation, a thermal head is in general use. In such a conventional thermosensitive recording material, there are usually employed in the thermosensitive coloring layer a colorless or light-colored leuco dye containing a lactone ring, a lactam ring or a spiropyran ring, and a color developer which induces color formation in the leuco dye upon application of heat by the reaction with the leuco dye, since it is capable of yielding clear images with minimized fogging.
Because of the capability of forming colored images by simple application of heat, such thermosensitive recording materials are widely used, not only for copying books and documents, but also for recording output information from computers, facsimile apparatus, telex and other information transmission and measuring instruments. Depending upon the recording mode, it will be more convenient if it is allowed to record particular data in a different color from the remainder on a thermosensitive recording material in order to display the particular data more distinctly from the remainder.
Recently, many trials have been made to attain recording with multiple colors by applying heat at different temperatures or by applying different quantities of thermal energy. Accordingly, a variety of multi-color thermosensitive recording sheets have been proposed.
A conventional multi-color thermosensitive recording sheet comprises a support material and two thermosensitive color-forming layers overlaying the support material, which color-forming layers are colored in different colors upon application of different thermal energies thereto respectively. One layer is referred to as, for example, a high-temperature color-forming layer and the other is referred to as, for example, a low-temperature color-forming layer. The low-temperature color-forming layer forms a color at a low temperature, while the high-temperature color-forming layer does not form a color at all at the low temperature, but forms a color at a high temperature which is higher than the low temperature, and the two colors are different from each other.
Such conventional multi-color thermosensitive recording sheets can be roughly classified into the following two types.
In one type, when a high-temperature color-forming layer is colored by application of heat at a high temperature, the color developed in the high-temperature color-forming layer is mixed with the color already developed in a low-temperature color-forming layer, so that a different color from the color in the low-temperature layer is produced in the high-temperature color-forming layer.
In the other type, when the high-temperature color-forming layer is colored, the color in the low-temperature color-forming layer is decolorized by a decolorizing agent, so that only the high-temperature color-forming layer is colored without the color of the low-temperature color-forming layer being mixed therewith.
Specific examples of the former type are disclosed, for instance, in Japanese Patent Publications No. 49-69, No. 49-4342 and No. 49-27708, and Japanese Laid-Open Patent Applications No. 48-86543 and No. 49-65239.
Specific examples of the latter type are disclosed, for instance, in Japanese Patent Publications No. 50-17865, No. 50-17866, No. 51-29024 and No. 51-87542, and Japanese Laid-Open Patent Applications No. 50-18048 and No. 53-47843.
The former type has the shortcoming that the practically developable color systems are limited to such combinations that the color developed at high temperature can overcome the color developed at low temperature, such as red (low temperature)--black (high temperature), and blue (low temperature)--black (high temperature).
In the latter type, there are no particular limitations to the combination of colors. However, a decolorizing agent capable of completely decolorizing the color developed in the low-temperature coloring color-forming layer (when developing a color in the high temperature color-forming layer) has not been discovered, although higher aliphatic alcohols, polyether, polyethylene glycol derivatives, nitrogen-containing compounds such as acetamide, stearamide, phthalonitrile, and amine derivatives such as guanidine derivatives are proposed as such decolorizing agents, so that the separation of the color in the high temperature color-forming layer from the color in the low temperature color-forming layer is incomplete. The result is that the color developed in the low temperature color-forming layer spreads and overlaps the color developed in the high temperature color-forming layer. In order to avoid such problem, it is necessary to overlay a high temperature color-forming layer, a decolorizing layer and a low temperature color-forming layer successively on a support material and to increase the thickness of the decolorizing layer so as to completely decolorize the color developed in the low temperature color-forming layer. However, when such a thick decolorizing layer is formed in the above thermosensitive recording sheet, a large quantity of thermal energy is required for color development in the high temperature color-forming layer by a printer or facsimile apparatus including a thermal head, and the color in the high temperature color-forming layer (located lowermost) cannot always be developed to its full extent in the high speed recording when a high speed recorder is employed, since quick heat transfer in the direction of the depth of the thermosensitive coloring layer is hindered by the depth of the layer.